1. Field of the Invention
The present invention relates to a sensor for measuring the concentration or presence/absence of carbon dioxide in respiratory gas through the nostrils or mouth of a living body. More particularly, the invention relates to a simple, compact sensor for measuring carbon dioxide in respiratory gas which can improve the accuracy in measurement and response.
2. Related Art
In general, when the concentration of carbon dioxide contained in respiratory gas from a living body is optically measured, the respiratory gas is caused to pass through a cylindrically-shaped airway adapter. An infrared ray is radiated onto the respiratory gas from a light-emitting element. A voltage corresponding to the amount of light which is absorbed by carbon dioxide contained in the respiratory gas is detected by a light-receiving element, thus measuring the concentration of carbon dioxide.
FIG. 16 shows the schematic configuration of an example of such a related capnometer. As shown in FIG. 16, one end 101a of an airway adapter 101—which is formed into a substantially cylindrical shape and through which respiratory gas passes—is to be connected to a tube inserted into a trachea of a patient. Another end 101b is to be connected to a Y piece of a respiratory circuit, such as a ventilator. An intermediate portion of the airway adapter 101 has a rectangular cross-sectional profile. Circular windows 101c, 101d are formed in respective, mutually-opposing surfaces of the intermediate portion such that the windows are concentrically aligned with each other. A sensor main unit 102 is removably attached to the intermediate portion of the airway adapter 101.
The sensor main unit 102 is formed into a rectangular-parallelepiped shape, and a U-shaped notch is formed in an intermediate portion of the sensor main unit 102. The intermediate portion of the airway adapter 101 is to be fittingly attached to the notch. Two mutually-opposing surfaces come into contact with the windows 101c, 101d of the airway adapter 101. A light-emitting element 103 emitting a infrared light is disposed on one side with reference to the notch formed in the sensor main unit 102.
A filter 104 for absorbing only light having wavelength to be absorbed by carbon dioxide and a light-receiving element 105 are disposed on the other side with reference to the notch formed in the sensor main unit 102. The light-emitting element 103 and the light-receiving element 105 are connected to a monitor main unit 107 via a lead wire 106. The intermediate portion of the airway adapter 101 can be removably attached to the sensor main unit 102.
In the related capnometer having the foregoing configuration, light emitted from the light-emitting element 103 enters the light-receiving element 105 by way of the window 101c, the respiratory gas in the airway adapter 101, the window 101d, and the filter 104. The quantity of light corresponding to the concentration of carbon dioxide is detected by the light-receiving element 105. An output signal from the light-receiving element 105 is input to the monitor main unit 107, where the concentration of carbon dioxide is displayed.
In the related-art example, the airway adapter 101 through which respiratory gas passes is attached to the sensor main unit 102. A related-art capnometer has a structure in which a sampling tube is connected to a sensor main unit disposed in the monitor main unit 107. In this case, one end of the sampling tube which aspirates part of respiratory gas is connected to the airway adapter 101 through which the respiratory gas passes. The other end of the sampling tube is connected to the monitor main unit 107. A pump is disposed in the monitor main unit 107, and respiratory gas is led to the sensor main unit disposed in the monitor main unit 107.
U.S. Pat. Nos. 5,099,836 and 5,335,656 describe other types of related respiratory gas sensors. U.S. Pat. No. 5,099,836 shows a partially cutaway top view shown in FIG. 17. As shown in FIG. 17, the inside of a tubular nasal cannula 201 is separated into a first separate chamber 203 and a second separate chamber 204 by a wall 202. One end of the wall 202 is connected to one side of an interior surface of the nasal cannula 201, and the other end of the same is connected to the other side of the interior surface, thus hermetically separating the two separate chambers 203, 204 from each other. Nasal tubes 205, 206 to be inserted into respective nostrils project in parallel with each other from an outer periphery of the nasal cannula 201. The inside of the nasal tube 205 is separated into passageways 205a, 205b by the wall 202 extending from the inside of the nasal cannula 201. Similarly, the inside of the nasal tube 206 is separated into passageways 206a, 206b by the wall 202. The passageways 205a, 206a are in communication with the first separate chamber 203, and the passageways 205b, 206b are in communication with the second separate chamber 204.
An oxygen gas supply tube 207 is connected to one end of the first separate chamber 203 of the nasal cannula 201. A tubing 208 is connected to one end of the second separate chamber 204 for sensing the user's breathing pressure. An oxygen cylinder is connected to the tube 207 through a pressure control valve (not shown). The tubing 208 is connected to a pressure transducer (not shown).
There may be a case where the tubing 208 is used as a sampling tube and connected to a carbon dioxide sensing monitor (not shown) to measure the concentration of carbon dioxide in respiratory gas.
In relation to the apparatus for detecting respiratory gas configured in the manner as mentioned above, the oxygen supplied from the oxygen cylinder through the tube 207 is fed to the nostrils through the first separate chamber 203, the passageway 205a of the nasal tube 205, and the passageway 206a of the nasal tube 206. Part of exhaled gas from the nostrils is discharged to the tubing 208 through the passageway 205b, 206b and the second separate chamber 204, and the breathing pressure and the concentration of carbon dioxide are detected.
As shown in FIG. 18, U.S. Pat. No. 5,335,656 relates to a nasal cannulae 301 which has a tubular body, is to be attached to the skin in the vicinity of the nose, supplies a treating gas to the nostrils, and measures the concentration of carbon dioxide in respiratory gas. A septum 302 is disposed in the nasal cannulae 301 to define an inhalation manifold 303 and an exhalation manifold 304. A flexible tubing segment 305 is connected to one end of the nasal cannulae 301 for supplying a treating gas to the inhalation manifold 303. A flexible tube segment 306 is connected to the other end of the nasal cannulae 301 for aspirating the exhaled gas from the exhalation manifold 304.
A nasal prong 307 is connected to the inhalation manifold 303 for supplying the gas to a first nostril. The exhalation manifold 304 is connected to and is communicated with a nasal prong 308 which is fitted to a second nostril to aspirate the exhaled gas. The segment 306 is connected to a carbon dioxide measurement sensor (not shown) which measures the partial pressure of carbon dioxide in the exhaled gas.
The related capnometer shown in FIG. 16 requires the airway adapter 101. The airway adapter 101 is connected to an endotracheal tube and a Y piece. Hence, difficulty is encountered in connecting the apparatus to a patient to whom no endotracheal tube is attached. Further, the apparatus is bulky, expensive, and complicated in construction. Further, the airway adapter 101 must be replaced, thus increasing operating costs. The light-emitting element 103 has hitherto involved power consumption of 1 W or more. Hence, the sensor main unit 102 becomes hot. If the apparatus is designed so as to come into direct contact with the skin, the apparatus will remain in contact with the skin for a long period of time for measurement, thus posing a risk of heat injury.
When the tubing shown in FIG. 17 is used, there arises a problem of the tubing is clogged with moisture in respiratory gas after long-term use. Further, the detection tube is disposable, thereby adding to operating costs. When the tubing is used as a sampling tube, there arises a time lag until the time carbon dioxide gas is detected, because the length of the tubing is usually 2 meters or more. Hence, detecting response becomes slow, which in turn deteriorates detection accuracy.
According to the related-art example shown in FIG. 17, if one of the pair of nostrils into which the nasal tubes 205, 206 are inserted has been clogged, the tube inserted in the thus-clogged nostril comes to sample air. The concentration of carbon dioxide gas in a sampled respiratory gas becomes about one-half the actual concentration in respiratory gas, which in turn may cause an error in measurement. Further, in the related-art example shown in FIG. 18, if the nostril into which the nasal prong 307 of the inhalation manifold 303 is inserted has been clogged, a treating gas cannot be supplied to a living body. In contrast, if the nostril into which the nasal prong 308 of the exhalation manifold 304 is inserted has been clogged, detection of carbon oxide gas becomes impossible.